Incandescent light sources produce a near black-body optical spectrum, are low cost and can have high reliability. However, such light sources have low efficiency, and are relatively large requiring large light fittings. High intensity discharge lamps are capable of producing high luminous flux from small arc sources and are suitable for projection applications and directional illumination but are bulky. Fluorescent lamps in which a gas discharge generates ultraviolet wavelengths which pumps a fluorescent material to produce visible wavelengths, have improved efficiency compared to incandescent sources, but similarly suffer from a large source size.
Light-emitting diodes (LEDs) formed using semiconductor growth onto monolithic wafers can demonstrate significantly higher levels of efficiency. The source size is defined by the area of LED die, and so in principle can be made of arbitrary size up to the size of the wafer. LEDs are particularly attractive as they offer high levels of efficiency with acceptable CIE Colour Rendering Indices (CRI). Organic light-emitting Diodes (OLEDs) promise similar benefits to wafer type LEDs, and can be formed on arbitrarily large substrates. In this specification LED refers to an unpackaged LED die (chip) extracted directly from a monolithic wafer, i.e. a semiconductor element. This is different from packaged LEDs which have been assembled into a package to facilitate subsequent assembly and may further incorporate optical elements such as a hemispherical structure which increases its size but increases light extraction efficiency.
In lighting applications, the light from the emitter is directed using a luminaire structure to provide the output directionality profile. The angular variation of intensity is termed the directional distribution which in turn produces a light radiation pattern on surfaces in the illuminated environment and is defined by the particular application. Lambertian emitters enable light to the flood a room with light. Such use of light can be inefficient as the light is distributed to parts of the room that may not require illumination, and also can be perceived as providing a wasteful and visually undesirable illumination source. Lambertian emitters can be perceived as providing a flat lighting environment lacking in sparkle which is unattractive to designers.
Non-Lambertian, directional light sources use a relatively small source size lamp such as a tungsten halogen type in a reflector and/or reflective tube luminaire, in order to provide a more directed source. Such lamps efficiently use of the light by directing it to areas of importance. These lamps also produce higher levels of visual sparkle, in which the small source provides specular reflection artefacts, giving a more attractive illumination environment. Further, such lights have low glare, in which the off-axis intensity is substantially lower than the on-axis intensity so that the lamp does not appear uncomfortably bright when viewed from most positions. However, such a lamp combined with its luminaire may have size of order 5 to 10 cm depth and further require a bulky low voltage supply. Such a lamp disadvantageously can require architectural design choices to be made requiring relatively deep recesses. Further, the materials cost of such bulky lamp systems can be high. Further, such sources have low efficiency compared to LED sources.
To further enhance usage efficiency, it would be desirable to be able to adjust the pointing direction of directional light sources. For conventional directional lamps, this typically requires a manual movement of the lamp. This can often be difficult because of the location and surface temperature of the lamp. The alignment of the lamp typically is thus typically set once at installation. To deal with the changing usage patterns in the room, multiple directional lamps are installed giving an inefficient over-illumination of a room.
The colour temperature and CRI of a lamp is an important factor in its use. The colour output is typically fixed at manufacture, and by its operating conditions, such as peak running temperature and lifetime. This means that the adjustment of the white point of a room is typically not possible through the period of a day, in order to match the demands of the activity. For example, it is reported that high colour temperatures can be used to encourage office worker efficiency while lower colour temperatures are well suited to more relaxed room usage, such as evening domestic lighting. Further, the eye is sensitive to small white point changes in an illuminated area so that illumination systems typically need to be selected to a fixed colour temperature in any given environment.
Natural lighting can produce time varying illumination, for example sunlight through a tree canopy providing relatively high resolution time varying patterns. If a room is illuminated by a single time varying source, then this will merely be seen as intensity fluctuations. It would be desirable if the time varying component of intensity varies in structure as opposed to globally. Such an approach would provide dappled illumination.
Liquid crystal display backlights typically use arrays of packaged LEDs or fluorescent lamps coupled to the panel by means of optical waveguiding elements combined with light recirculating films. Further, the directionality of the display can be enhanced by means of brightness enhancement films, such as prismatic layers. Such systems can be 5 mm or less thickness. However, the structures are designed to distribute light uniformly over a large area from small sources, and as such do not function well as compact directional illumination sources. It would be desirable to provide high efficiency directional illumination for example for use as a privacy filter, or to provide high brightness outdoor display function.
Directional LED elements can use reflective optics (including total internal reflective optics) or more typically catadioptric (or tulip) optic type reflectors, as described for example in U.S. Pat. No. 6,547,423. Catadioptric elements employ both refraction and reflection, which may be total internal reflection or reflection from metallised surfaces. A known catadioptric optic system is capable of producing a 6 degree cone half angle (to 50% peak intensity) from a 1×1 mm square light source, with an optical element with 13 mm final output diameter. The increase in source size arises from conservation of brightness (étendue) reasons. Further, such an optical element will have a thickness of approximately 11 mm, providing a bulky illumination apparatus. Increasing the cone angle will reduce the final device area and thickness, but also produces a less directional source.
US2008/0157114 describes a replication system in which multiple hemispherical structures are embossed in a single replication step.
White LED lighting sources can be of separate spectral bands such as red, green, blue and yellow, each created by a separate LED element. Such sources enable users to resolve the separate colours, and with the separation of the sources in the lamp which can create coloured illumination patches. It would be desirable if the sources were homogenized so that their separation was less than the visual resolution limit.
Alternatively, white LEDs can be blue emitters combined with yellow phosphor to produce the desired white output. To achieve the required total luminous output, it is well known that an array of LEDs can be used. For example, a 4×4 array of 1 mm devices separated by 0.3 mm will give a total array size of 4.9×4.9 mm with a Lambertian output. Such a source suffers from the need to have complex heatsinking and cooling arrangements, leading to high device cost and thickness. If this is used in a 6 degree cone angle catadioptric optic, then the output aperture and thickness may be expected to approach 75 mm, similar to that of incandescent lamps. If this is combined with the heat sink, it is clear that directional LED arrays will disadvantageously have substantial bulk. In order to reduce thickness and thus materials cost, such arrays may use one catadioptric optic per LED rather than over a group of LEDs. Prior art systems such as ‘pick and place’ machines align each LED to each catadioptric optic followed by packing of the catadioptric optic illuminators.
By way of comparison, prior art light sources use small numbers of high flux illumination sources. If one of the sources fails, this produces a noticeable reduction in output flux. In directional sources, this will result in missing areas of illumination. It would be desirable to reduce the visibility of failed sources, thus improving system yield and lifetime. High efficiency prior art LED sources have very high luminance levels from small areas of source. Such high brightness can be uncomfortable to look at directly.
By way of comparison, to produce acceptable illuminance for directional lighting systems, known LEDs are of macroscopic scale; that is they use LEDs typically of size 1×1 mm or greater. Each LED is arranged with a catadioptric optical element to reduce the output solid angle of illumination. Known optical elements have output apertures of size of 13 mm or greater, and thickness of 11 mm or greater for a 6 degrees cone half angle. In this specification, light-emitting element width is the maximum width along one edge for the light emitting regions of square or rectangular light-emitting elements. Light emitting element diameter is the maximum diameter of light-emitting regions for circular or elliptical light-emitting elements.